1. Field of the Invention
The present disclosure relates to carbon fiber composites formed using asphaltene-based binders.
2. Description of Related Art
Carbon fiber-reinforced composites are known in the art. Various precursors are used to produce carbon fibers of different morphologies and different specific characteristics. Typical precursors include polyacrylonitrile (PAN), cellulosic fibers (e.g., rayon, cotton), petroleum or coal tar pitch, and certain phenolic fibers. Carbon fibers are manufactured by the controlled pyrolysis of these precursors in fibrous form. There are typically three or four successive stages in the conversion of a precursor into carbon fiber. First, depending upon the precursor material, it can be subjected to spinning and drawing to convert the material into filaments. Next, the filaments are subjected to oxidative stabilization/thermosetting by stretching and simultaneously oxidizing the material at a predefined temperature. This treatment prepares the fibrous precursor to undergo additional processing at higher temperatures without changing the fiber form. The fibers are carbonized at elevated temperatures, typically 1500-1800° C., in an inert atmosphere (e.g., N2). During this process the non-carbon elements are volatilized to yield carbon fibers. The carbon yield will depend on the starting material. Typically, a PAN precursor will give a yield of about 50% of the mass of the original PAN. Further heating up to 2500° C. will improve the ordering and orientation of the carbon, resulting in a higher percentage of graphitic structures in the fiber. Finally, the fiber may or not be subjected to a surface treatment to improve handling properties and increase bonding strength to adhesives.
Carbon fiber comes in several forms and has found wide applicability, primarily because carbon fibers have extremely high tensile strength per unit weight, anisotropically along the length of the fiber. The basic form available is a bundle of small diameter (5-10 microns) filaments containing from 1,000 to 24,000 individual filaments, and is called a tow. When mixed with an adhesive binding agent, these tows can be used as is, for winding cylindrical objects, such as high pressure tanks and cylinders. Or, the tows can be woven into many fabric designs and used for structural flat plates or curved shapes. The tows can be chopped into small lengths to produce a molding compound for more complex shapes. In each case, the carbon fiber provides a significant increase in the structural properties of the resultant product.
Conventional carbon fiber-reinforced composites consist of carbon fiber preforms impregnated with either a thermoplastic or thermosetting polymeric binder. Thermoplastic resins may include such materials as acrylonitrile butadiene styrene (ABS) and polyether ether ketone (PEEK), whereas thermosetting resins are typically epoxy, polyester, or phenolics. The carbon fiber provides the primary reinforcement for the composite and represents the largest volume fraction of the composite. The polymeric binder, which accounts for about one third of the composite volume, holds the composite together. As the strength of the composite is proportional to the volume fraction of each component, and given that carbon fiber typically has mechanical strength two orders of magnitude greater than polymeric binders, a carbon fiber reinforced composite provides far superior mechanical properties. On the other hand, most polymeric binders begin to soften or breakdown around 150-300° C., depending upon the material, and thus the use of polymeric binders impart thermal limitations on the composite.
With carbon fiber filaments exceeding 1,000,000 psi tensile strength now available, the weak link in a polymeric carbon fiber reinforced composite is the binder. Typical polymeric resin binders typically have tensile strengths of less than of 15,000 psi and a continuous use temperature limited to 200° C. or less. More recently, carbon-carbon composites prepared using carbonized thermosetting resins, carbonized pitch, or chemical vapor infiltration (CVI), have been employed. Pitch, bitumen, and asphalt are crude and inexpensive materials that are a by-product of petroleum refining. However, there is still a need in the art for improved composite products and manufacturing methods.
Asphaltenes are thought to consist of highly-ordered and complex aromatic ring structures typically containing small amounts of hydrogen, nitrogen, oxygen, sulfur, and/or heavy metals in addition to their primary constituent, carbon. They are large, planar, hetero-atom containing molecules that lend themselves to pi-pi bond stacking. Asphaltenes are a distinct chemical component of asphalt, which can be isolated from the resins in asphalt or pitch based on solubility by solvent extraction and other methods. They occur widely in heavy oil-producing formations, and are the non-melting (i.e., burns before melts) solid component of crude oil, giving such crude oils their color. For example, heavier, black-oil crudes, such as those found in tar sands, will typically have a higher asphaltene content.